This study examined stool and urine energy losses in obese versus lean individuals on two standard (2400 kcal/d and 3400 kcal/d each given over 3 days) diets. Preliminary results from an initial 14 lean and 9 obese individuals failed to show any difference in stool or urine energy loss. The range of calories stool was large, varying from 2 to 9 % in both groups. Lean individuals absorbed more (had less stool energy loss) on the higher calorie diet compared to obese individuals. Percent changes in the major gut phylotypes, Firimutes and Bacteroidetes, were associated with changes in nutrient load. Percent of Firmicutes increased with nutrient load while Bacteroidetes decreased. In addition, in lean individuals, these changes in the major phylotypes were associated with stool energy loss, such that a 20% increase or decrease in Firmcutes/Bacteroidetes was associated with approximately 150 kcal/day difference in stool energy loss. The endocannabinoid system has recently been implicated in regulation of gut permeability and thus nutrient absorption. We investigated whether the concentrations of the circulating endocannabinoids, anandamide (AEA) and 2-arachidonoylglycerol (2-AG) are associated with stool calories loss. We found that higher nutrient load reduced AEA concentrations in lean but not obese individuals. AEA was also associated with greater nutrient absorption (less calories loss in stool) in obese but not lean individuals. This may indicate a role for the endocannabinoid system in intestinal nutrient absorption. Based on our preliminary work, we have designed a new study to confirm and extend these findings. In this study, stool energy loss and changes in gut microbiota will be measured during over and underfeeding (150 and 50% based on calculated weight maintaining calories). Following this volunteers will be randomized to receive the antibiotic oral vancomycin versus placebo. Oral vancomycin is not absorbed in health individuals and will selectively change the gut bacterial population. This will allow us to investigate whether there is an association between changes in gut microbiota and nutrient harvest (as measured by stool energy loss). We will also investigate how changes in the gut microbiota affect glucose tolerance, energy expenditure and substrate oxidation.